Magnetron pulse current regulator



Dec. 17, 1968 c KENNY 3,417,343

MAGNETRON PULSE CURRENT REGULATOR File d Aug. 9. 1967- PULSE SOURCE 22 United States Patent 3,417,343 MAGNETRON PULSE CURRENT REGULATOR Charles R. Kenny, Somers, N.Y., assigior to General Precision Systems Inc., a corporation of Delaware Filed Aug. 9, 1967, Ser. No. 659,380 Claims. (Cl. 331-87) ABSTRACT OF THE DISCLOSURE The amplitude of the pulse current to a magnetron load is sensed and utilized to establish a potential which is proportional thereto. A fixed reference potential is established and compared with the first-mentioned potential, and the difference therebetween utilized to control the screen grid voltage of a shunt switch tube in such manner that the peak forward current applied to the magnetron maintains a substantially constant magnetron peak current output.

A brief summary of the invention This invention relates to the control of the pulse current flowing in the forward direction in a magnetron and, more particularly but by no means exclusively, to such pulse current control in a magnetron which is operating in a shunt modulator circuit of an airborne, Doppler radar receiver-transmitter unit.

It is believed well-known in this art that the frequency of the oscillating output of a magnetron varies with the forward current, with the degree of this variation being commonly referred to as the Pushing Factor. Shunt modulators of the type referred to hereinabove that utilize a beam tetrode as a shunt electronic switch do so with the expected advantage of constant current generation which is typical of pentodes and tetrodes. In the ideal case, the shunt switch current, and accordingly the forward magnetron current is essentially independent of switch tube anode voltage, while the constant shunt anode current is determined by the respective control grid and the screen grid potentials.

In shunt modulators which employ pentode and tetrode switch tubes it is common practice to drive the control grid with the pulsed switching potential, and to apply a fixed potential to the screen grid to achieve a desired anode peak current. As a magnetron ages, however, the potential required to produce a given forward current increases. With an ideal modulator, that is, with a constant current switch, the change in magneton voltage drop would not affect the forward current. In a practical modulator, however, the switch tube is not a constant current device, whereby variations in such parameters as supply voltage, drive voltage, screen voltage, magnetron voltage and component variations with age cause short and long term variations in the peak forward current to the magnetron. These variations in magnetron current are in turn causative of degradation of the spectrum of the RF output, variations in power, and reduced life expectancy of the magnetron.

It is, accordingly, a primary object of this invention to provide a regulator means for operating a magnetron at constant optimum peak current with resultant significant improvement in the magnetron output characteristics.

Another object of this invention is to provide magnetron operating regulator means which significantly increase magnetron life expectancy, whereby long periods of satisfactory, maintenance-free magnetron operation are insured.

A further object of this invention is to provide magnetron operating regulator means which are particularly, but by no means exclusively, adapted to the operation of a magnetron in a shunt modulator circuit of the nature utilized in receiver-transmitter units of airborne, Doppler radar devices.

As currently preferred, the magnetron pulse current regulator of the invention comprises pulse current transformer means which functions to sense the pulse current amplitude to the magnetron load and enable grounding of the magnetron anode, that is, the body of the magnetron. The sensing transformer is preferably designed to operate with a one-turn primary, whereby the lead to the magnetron cathode may be utilized as the primary by passing the lead through the core of the transformer. In addition, the use as above of the pulse current transformer provides the advantages of high voltage isolation and low shunt capacity.

Thus pulse peak potential induced in the secondary winding of the pulse current tansformer will be proportional to the primary peak current when the transformer secondary winding is properly loaded. This pulse voltage is detected by semiconductor peak rectifier and filter means, and a DC. potential is obtained which is proportional to the peak primary current of the transformer. This DC. potential is in turn compared to a reference potential through the use of a differential amplifier with the former being provided by a Zener diode, and the differential amplifier being constituted by a common circuit using two transistors. As a result, the difference between the detected and reference voltages will be a measure of the departure of the peak primary current from the desired or reference value.

The magnetron pulse current regulator of the invention makes use of the output of this differential amplifier to control the potential of the screen grid of the tetrode switch tube. To this effect, a triode D.C. amplifier is used with the differential amplifier output being directly connected to the triode grid, and the screen of the tetrode directly connected to the triode anode. The thusly formed hybrid amplifier, and the network used to provide low potential power to the differential amplifier make the regulator of the invention particularly useful in applications wherein power is limited. Alternatively, transistor means could of course be utilized for this final inverting amplifier with attendant reduction in regulator power requirements.

The output of the differential amplifier is amplified by the DC. amplifier and the polarity thereof inverted, whereby the screen voltage is maintained at a value which results in a minimum departure of peak primary magnetron current from the desired value.

The above and other objects and significant advantages of my invention are believed made clear by the following detailed description thereof taken in conjunction with the accompanying drawing wherein:

The figure is a schematic diagram of a magnetron pulse current regulator constructed in accordance with the teachings of this invention.

Referring now to the figure, a magnetron is indicated generally at 10 and is connected as shown in a shunt modulator circuit as indicated generally at 12. More specifically, the shunt modulator circuit may be seen to comprise a shunt switch tetrode 14, high voltage supply means 16, choke means 18, a condenser 20, a restorer tube 22, and pulse source means 24, respectively, which are arranged as indicated to form the said shunt modulator circuit.

The shunt switch tetrode or switch tube 14 is arranged to be normally nonconductive, and restorer tube 22 functions to confine the plate 26 of the condenser 20 to a potential range which is negative with respect to ground. As a result, the application of a pulse from the pulse source means 24 to the control grid 28 of the switch tube 14 will render the latter conductive and suddenly reduce the plate 29 of the condenser 20 to near ground potential with attendant dropping of the condenser plate 26 to a negative potential nearly corresponding to that of the high voltage supply means 16 as, for example, 4,000 volts. The condenser 20 will thus discharge through the magnetron 10 to produce the characteristic oscillating pulse output.

The peak forward driving current to the magnetron 10 may of course vary due to numerous causes as set forth in detail hereinabove. In order to maintain this peak forward driving current substantially constant, the magnetron pulse current regulator of the invention comprises a sensing transformer as indicated generally at 30 which is coupled magnetically as shown to the lead of the cathode 34 of the magnetron 10. More specifically, the sensing transformer 30 comprises primary windings 38 and secondary windings 40; and the secondary windings 40 constitute the lead to the cathode 34 of the magnetron 10 which is passed through the window of the non-illustrated core upon which the sensing transformer secondary winding 40 is wound. Thus is provided a one-turn primary winding which is necessary to the reduction of shunt capacity across the magnetron circuit.

A rectifying circuit is indicated generally at 42 and is constituted by diodes 44 and 46 which are arranged as shown, and a resistance 48 is connected across the transformer winding 40 and utilized to establish the basic scale factor of the secondary peak-peak voltage vs. primary peak-peak current.

Since only peak forward current sensing is desired here, and since the potential of transformer secondary winding 40 is a measure of peak to peak primary current, the function of diode 44 is explained in somewhat more detail as follows. In actual applications of the regulator of the invention, a large reverse current is induced for a short time due to the use of non-illustrated tail biting or shunt capacity discharge circuit means. Accordingly, diode 44 is used to prevent the saturation of the non-illustrated core of transformer 30 which might otherwise result from large reverse current flow, and loss of linearity in forward current transformation. It is of interest to note that the peak voltage induced in secondary winding 40 due to the primary forward current is, to some small extent, in error as a result of the inability of sensing transformer 30 to pass a direct current. However, with the low fixed duty ratio pulses in the order of which are prevalent here, this error is maintained constant, and quite small. Thus, the diode 44 can function to allow negative voltages in the order of 5% of the peak forward voltages to appear for purposes of retaining the substantial linearity of operation of the sensing transformer.

Series connected resistance 50 and capacitance 52 are connected across the transformer winding 40 and function to restore the pulse wave form to a precise replica of the primary current pulse.

A voltage divider is generally indicated at 56 and is constituted by the potentiometer 58 and a resistor 60 which are connected as shown across the output of the rectifier means 42. Adjustment of the potentiometer 58 will function as a vernier adjustment of the scale factor and thus make unnecessary the simultaneous adjustment of resistance 50 and equalizing capacitor 52 which would be required were adjustment resistance 48 utilized for the scale factor adjustment function.

A differential amplifier is indicated generally at 62 and is constituted by transistors 64 and 66, respectively, connected as shown with the base 68 of the former being connected to receive a selected portion of the voltage from the voltage divider means 56.

A Zener diode divider circuit is indicated generally at 70 and comprises Zener diodes 72 and 74 connected as shown to the base 76 of the transistor 66; and a resistance 75 is included to provide the requisite series ballast resistance for the said Zener diodes. A voltage supply 77 effects the provision of a reference potential from the divider circuit 70.

Amplifier means are indicated at 78 and are connected in the control circuit as shown to control the potential of the screen grid 80 of the switch tube means 14.

In operation, the magnetron pulse current regulator of the invention functions to maintain the peak current output of the magnetron 10 at a substantially constant level through the sensing of the magnetron peak current outputproduced as above through the discharge of the condenser 20 through the magnetron 10 by the sensing transformer means 30 which, as described hereinabove, are coupled magnetically to the cathode lead of the magnetron 10. More specifically, the peak current is sensed by the sensing transformer means 30 as a voltage induced in the secondary windings 40 which, as described above, are connected as shown to the rectifying circuit 42. The resultant voltage output of the rectifying circuit 42 will thus be proportional to the induced transformer voltage as Well as the peak magnetron current; and this rectifying circuit voltage output is applied in turn across the voltage divider means 56.

Application of a selected portion of this rectifying circuit voltage output to the differential amplifier means 62 is accomplished by the application thereof to the base 68 of the transistor 64 which, as described above, constitutes one half of the said differential amplifier. Concurrently, a fixed potential is applied to the base 76 of transistor 66 from the Zener diode divider circuit 70, whereby the output of the differential amplifier 62 will consist of the difference between the rectified potential applied to transistor 64 and the fixed reference potential applied to transistor 66. This differential amplifier output is applied to the DC. amplifier 78 with resultant amplification and polarity inversion thereof, and the output of the amplifier 78 is in turn applied to the screen grid 80 of the switch tube means 14 to control the potential thereof and maintain the screen grid voltage at a value which results in a minimum departure of the peak current from the derived value. Thus, the forward current through the magnetron 10 is maintained substantially constant in full accordance with the primary object of the invention.

While this invention has been shown and described in connection with a single preferred embodiment, it is apparent that various changes and modifications, in addition to those mentioned above, may be made by those who are skilled in the art without departing from the basic features of the invention. Accordingly, it is the intention of the applicant to protect all variations and modifications within the true spirit and valid scope of this invention.

What is claimed:

1. In a magnetron pulse current regulator for controlling the peak forward current applied to a magnetron through shunt switch means to maintain the peak current output of the magnetron substantially constant,

means for sensing the pulse current applied to the magnetron load and establishing a potential proportional thereto,

means for establishing a fixed reference potential,

means for comparing said fixed reference potential with the first-mentioned potential to determine the difference therebetween and for providing an output proportional thereto, and

means to control the peak forward current applied to the magnetron through said shunt switch means in accordance with said potential difference output to maintain the peak current output of the magnetron substantially constant.

2. In a magnetron pulse current regulator as in claim 1 wherein,

said current sensing and potential establishing means comprise pulse current transformer means coupled magnetically to the magnetron cathode lead, and rectifier means connected to the secondary of said transformer means for establishing a voltage proportional to the magnetron pulse current. 3. In a magnetron pulse current regulator as in claim 1 wherein,

said means for comparing said potentials comprise differential amplifier means and means to apply each of said potentials thereto. 4. In a magnetron pulse current regulator as in claim 1 wherein,

said shunt switch means comprise switch tube means,

and said means to control said switch tube means comprise amplifier means connected to said potential difference output and the screen grid of said switch tube means and operable to control the potential of the latter in accordance with magnitude of the former. 5. In a magnetron pulse current regulator as in claim 2 wherein,

said means for comparing said potentials comprise differential amplifier means, and means to apply each of said potentials thereto. 6. In a magnetron pulse current regulator as in claim 5 wherein,

said shunt switch means comprise switch tube means,

and said means to control said switch tube means comprise amplifier means connected to said potential difference output and the screen grid of said switch tube means and operable to control the potential of the latter in accordance with magnitude of the former. 7. In a magnetron pulse current regulator as in claim 6 wherein,

said pulse cur-rent transformer means comprise a one turn primary which is constituted by the magnetron cathode lead being passed through the transformer means core. 8. In a magnetron pulse current regulator as in claim 6 wherein,

said means for establishing a fixed reference potential comprise a Zener diode divider circuit, and said differential amplifier means comprise a two transistor circuit with means to apply the first-mentioned potential to the base of one of said transistors and said fixed reference potential to the base of the other of said transistors. 9. In a magnetron pulse current regulator as in claim 8 wherein,

said amplifier means comprise a triode D.C. amplifier arranged so that the differential amplifier is connected directly to the triode grid and the triode anode is connected directly to the screen grid of the switch tube means. 10. In a magnetron pulse current regulator as in claim 8 wherein,

said amplifier means comprise a transistor.

JOHN KOMINSKI, Primary Examiner.

US. Cl. X.R. 33030; 328-67 

